Hyperthermia is the heating of living tissue for therapeutic purposes. Hyperthermia has been used as a method of treating cancer by means of raising the temperature of a tumor locally, the region of the body in which the tumor is located, or the whole body. It has long been known that high heat can contribute to the natural regression and/or remission of tumors. Because of its effect on cancer cells, hyperthermia may be used as an independent therapy or in conjunction with other cancer therapies, such as radiation, surgery, chemotherapy, and immunotherapy to enhance the effectiveness of these therapeutic modalities.
Current hyperthermia techniques used in cancer therapy include regional perfusion with heated fluids, microwave heating, fluid immersion, low frequency (RF) current fields, and ultrasound. Three of the most common types of currently used hyperthermia techniques involve radio frequency, microwaves and ultrasound. Radio frequency and microwave equipment may be used for local, regional and whole body heating. Ultrasound can be used for local and regional heating. Perfusion, the passing of a heated fluid through a limb, is limited to treatment of a limb. Immersion techniques involve immersing the body in a hot wax or hot water solution.
Hyperthermia systems have been developed utilizing direct contact microwave applicators. The depth of penetration of the microwave energy is frequency-dependent, and penetration is also a function of tissue composition and anatomical structure. The design of the microwave applicator influences the thermal distribution. In addition, sharp changes in patient contour within the treatment area (as in the head and neck) will have a strong influence on the thermal distribution.
In hyperthermia treatment systems such as radiofrequency, microwave and ultrasound, healthy normal tissue is heated as well as the tumor cells. Since normal healthy cells can be destroyed by elevated temperatures as well as cancer cells of a tumor, it is important during the hyperthermia treatment to maintain the temperature of the healthy tissues below the point in which damage is likely to occur while maintaining the tumor at elevated temperatures necessary for treatment. In such hyperthermia treatment techniques the temperature in the tumor will exceed the temperature of the surrounding healthy tissue, since the healthy tissue is cooled somewhat by the flow of blood through the entire body. Typically, tumors are not cooled by the flow of blood. Hyperthermia treatment involves the raising of the tumor temperature to a temperature on the order of 45.degree. C. for a prescribed period of time in a course of which treatment cancer cells (which normally cannot effectively withstand these temperatures) are damaged. During treatment, an effort is made to keep normal tissues at lower temperatures. Typically, tumors have a poor blood flow system as present in normal healthy tissue which carries off the heat of hyperthermia treatment. Healthy tissue is characterized by a developed blood vessel network and normal physiological response to heat, a phenomenon known as vasodilation where the blood may increase threefold after five minutes of heating. By way of contrast, tumors typically are characterized by a damaged blood vessel network and a collapsing blood flow during heating.
Hyperthermia treatment systems have been developed which operate only with a single microwave applicator and multiple temperature sensors. In such systems, only one sensor is used to actively control the hyperthermia system; this sensor is implanted in the tumor and provides temperature information to a computer for feedback and control of microwave power level and applicator. The other implanted temperature sensors perform a passive monitoring function, assisting the hyperthermia system operator in decision making during the treatment.
Hyperthermia systems have also been developed which employ up to twelve applicators, but independent power controls are not available for each applicator to optimize the heating pattern. These systems generally utilize only a single non-invasive temperature sensor.
Another type of hyperthermia system has been developed, as an annular phase array system, and it has up to eight individual microwave applicators and eight temperature sensor probes. As in other existing systems, the system has no independent power control for each applicator and only one of the temperature sensors planted within the tumor performs any active control function.
A need has thus arisen for a method and apparatus for optimizing the heating pattern in a microwave or ultrasound hyperthermia system for controlling the power input to single and multiple applicators in response to temperature control information from sensors detecting the temperature within a tumor and the surrounding normal tissue.